The dye-sensitized solar cell introduced by Gratzel et al. in 1991 is a wet solar cell with working electrodes formed of a porous titanium oxide film spectral-sensitized with a ruthenium complex and reported to have performances equivalent to those of a silicon solar cell (see Non-Patent Document 1 below). This approach has received attention because of its advantages that it enables the use of an inexpensive oxide semiconductor such as titania without purifying the semiconductor to a high purity and thus can provide an inexpensive dye-sensitized solar cell, which can convert light substantially in the whole wavelength region to electricity because of its broad dye absorptivity. However, the known ruthenium complex dye absorbs the visible light but little the infrared light of a wavelength of 700 nm or longer and thus is low in photoelectric-conversion capacity in the infrared region. Therefore, in order to further enhance the conversion efficiency, a development of dye has been demanded, which can absorb not only the visible light but also the infrared light.
Meanwhile, black dye can absorb light of up to 920 nm but is small in light absorption coefficient and thus needs to increase the amount to be adsorbed on a porous titanium oxide film in order to obtain a higher electric current value. Various approaches have been made to increase the amount to be adsorbed on a porous titanium oxide film. Generally, the amount can be increased by increasing the thickness of the film (see Non-Patent Document 2 below). As the film thickness is increased, the conversion efficiency can not be increased significantly due to the occurrence of reductions in open circuit voltage and FF (fill factor) resulting from an increase in reverse electron transfer and a decrease in the electron density in the film.
There is a report that a solar cell is produced using a complex containing an imidazophenanthroline ligand, which, however, can not obtain a sufficient conversion efficiency (see Patent Document 1 below).
Non-Patent Document 1: “NATURE” (Great Britain), Vol. 353, page 737 by B. O' Regan and M. Gratzel
Non-Patent Document 2: “Journal of American Chemical Society” (U.S.A) Vol. 123, page 1613, by M. Gratzel
Patent Document 1: WO2007/006026